1. Field of the Invention
The present invention relates to a MOS-type semiconductor device in which MOSFET is formed on a semiconductor layer provided in a thin wall-like fashion on an insulating film. More specifically, the present invention relates to a field effect transistor with a thin wall-shaped semiconductor body and double gates on both sides of the wall-shaped body (to be abbreviated as Fin-MOSFET hereinafter), and a method of manufacturing the transistor.
2. Description of the Related Art
Recently, Fin-MOSFET is attracting much attention, in which a silicon layer on the SOI wafer is vertically processed into a thin wall-like plate shape (i.e., fin-shape) as a body (i.e., channel region) of a MOSFET to be formed on an insulating layer. (See X. Huang et al., IEEE ED vol. 48, p 880 (2001)).
In the case where a body of an elemental device (i.e., transistor) is made of an extremely thin silicon layer such as in a Fin-MOSFET, to secure fast operation of the device, it is essential to reduce the electrical resistance of the source-drain regions which are to be formed around the opposing ends of the thin-body. In order to achieve this, there is proposed, for example, a method of forming layers of a metallic compound (i.e., silicide) by making a part of the source-drain regions react with a metallic substance. However, to take a full advantage of the silicide formation (i.e., silicidation), it is also necessary to reduce the contact resistance emerging at the interface between silicon and metallic silicide to a sufficiently low level, otherwise, the current flow into the silicide layers will be impeded at the interface, nullifying the reduction of the sheet resistance by the silicidation and thus impairing the fast device operation.
The contact resistance created at the interface between silicon and metallic silicide is a result of the Schottky barrier formed between them. It is a basic property of the Schottky barrier that, for one specific silicide, the sum of the Schottky barrier φn for electrons and the Schottky barrier φp for holes is always equal to the band gap Eg=1.1 eV (in the case of Si). Consequently, in the manufacture of a CMOS circuit, when it is designed to reduce the contact resistance for a transistor of one polarity (say, n-MOSFET), the Schottky barrier for a transistor of the opposite polarity (i.e., p-MOSFET) inevitably increases.
In the case of CMOS circuits, once the contact resistance of a transistor of either one of the polarities is increased and its operation is slowed down, the whole signal processing is hindered because the speed of the entire circuit is limited by the operation of the slower transistor, no matter how fast is the operation of the transistor of the opposite polarity. Under these circumstances, it is imperative to chose, as the siliciding material, a material that has similar φn and φp values. As a result, a Schottky barrier at a level of substantially a half of that of Eg is utilized for MOSFETs of both polarities.
It should be noted here that the contact resistance depends precipitously on the Schottky barrier height. For example in the case where a silicide layer is formed in a high-concentration diffusion layer having an impurity concentration of about 1020 cm-3, if the Schottky barrier height is decreased by 0.1 eV, the contact resistance is reduced nearly by one order of magnitude. For this reason, the Schottky barrier heights of about 0.5 eV, that are compulsively employed for both n and p polarities, is a great obstacle to the reduction of the contact resistance of a CMOS circuit to achieve a high-speed operation.
As described above, to take a full advantage of the fast device operation made possible by the Fin-MOSFET with the silicide layers on the source drain regions, it is necessary to reduce the contact resistance between the semiconductor layer that gives rise to source-drain regions and the silicide layer. However, in the manufacture of CMOS circuits, when it is designed to reduce the contact resistance for either one of the polarity, the Schottky barrier height of the opposite polarity inevitably increases, resulting in the increase in the contact resistance.